Infrared detectors are used in a variety of applications to provide an electrical output which is a useful measure of the incident infrared radiation. For example, quantum detectors are one type of infrared detector that are often used for night vision purposes in a variety of military, industrial and commercial applications. Quantum detectors generally operate at cryogenic temperatures and therefore require a cryogenic cooling apparatus. As a result, quantum detectors that operate at cryogenic temperatures can have a relatively complex design and generally consume significant amounts of energy.
Another type of infrared detector is a thermal detector. Thermal detectors are typically uncooled and therefore generally operate at room temperature. One type of thermal detector that has been developed and is becoming increasingly popular is a microbolometer-based, uncooled focal plane array. A focal plane array generally includes a plurality of pixel structures, each of which includes a bolometer disposed upon a common substrate. Each bolometer includes a transducer element that has an electrical resistance that varies as a result of temperature changes produced by the incident infrared radiation. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained. Since the design of a bolometer-based uncooled focal plane array is generally less complex than cryogenically cooled quantum detectors and since these uncooled focal plane arrays generally require significantly less energy than cryogenically cooled quantum detectors, bolometer-based uncooled focal plane arrays are being increasingly utilized.
Each pixel structure of a conventional uncooled focal plane array has a bolometer that includes an absorber element for absorbing infrared radiation and an associated transducer element having an electrical resistance that varies as its temperature correspondingly varies. In operation, infrared radiation incident upon the absorber element will heat the absorber element. Since the absorber element and the transducer element are in thermal contact, the heating of the absorber element will correspondingly heat the transducer element, thereby causing the electrical resistance of the transducer element to change in a predetermined manner. By measuring the change in electrical resistance of the transducer element, such as by passing a known current through the transducer element, a measure of the incident radiation can be obtained.
In order to permit the bolometer to be responsive to changes in the incident infrared radiation, the bolometer is generally designed to minimize thermal loss to the substrate. Thus, the bolometers of conventional focal plane arrays have separated the absorber and the transducer elements from the substrate so as to substantially thermally decouple the relatively massive substrate from the pixel. In this regard, each bolometer generally includes a thermal isolation structure (TIS) having two or more legs that support the absorber and transducer elements above the substrate, with the length of the legs scaling directly with the level of thermal isolation between the absorber/transducer and the substrate. The TIS connects the absorber and transducer elements to pillars or the like that support the absorber and transducer elements above the substrate. In conventional bolometers, however, the legs of the TIS generally do not extend underneath the absorber and transducer. Instead, the legs typically extend out from edge portions of the absorber and transducer. This is a drawback in conventional bolometers because for a given pixel structure size, the available area must be divided between the TIS, and the absorber and transducer, which limits the length of the legs and, in turn, the thermal isolation of the bolometer.
In bolometers where the absorber and transducer elements are separate layers, in order to provide thermal contact between the absorber and the transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and the transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649 and 5,367,167 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference.
In order to further improve the performance of conventional pixel structures, each bolometer can include a reflector disposed upon the surface of the substrate underlying the absorber and transducer elements. As such, infrared radiation that is incident upon the bolometer, but that passes through and is not absorbed by the absorber element, will be reflected by the reflector back towards the absorber element. At least a portion of the reflected radiation will therefore be absorbed by the absorber element during its second pass through the absorber element, thereby increasing the percentage of the incident radiation that is absorbed by the absorber element.
In operation, infrared radiation incident upon the pixel structure will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. In order to monitor the change in resistance of the transducer element and, therefore, to indirectly measure the infrared radiation incident upon the bolometer of the pixel structure, circuitry is generally formed upon the underlying substrate. The circuitry is generally electrically connected to the transducer element via a pair of conductive paths or traces defined by or upon the legs, pillars or the like that support the absorber and transducer elements above the surface of the substrate. By passing a known current through the transducer element, the change in electrical resistance of the transducer element can be measured and a corresponding measure of the incident infrared radiation can be determined.
While bolometer-based focal plane arrays having a plurality of pixel structures as described above are extremely useful, these conventional focal plane arrays have several disadvantages. Most notably, for the bolometer of each pixel structure, the characteristics of the absorber element and the transducer element cannot be separately optimized since the absorber and transducer elements are included within the same multilayer structure that is supported above the surface of the substrate. In this regard, in order to optimize the performance of the bolometer of a focal plane array, the absorption characteristics of the bolometer should be maximized while the thermal loss to the substrate should be minimized. In particular, the absorption characteristics are preferably maximized and the thermal loss to the substrate is preferably minimized in order to increase the sensitivity and, thus, thermal resistance, of the bolometer.
Since the pixel structures are generally disposed in an array, it is desirable that the absorber layer of the bolometer of each pixel structure be as large as possible and approximate the overall size of the pixel structure as closely as possible in order to maximize the fill factor. Unfortunately, the legs, pillars or the like that support the absorber and transducer elements of conventional bolometers above the surface of the substrate generally extend from edge portions of the absorber and transducer elements to the substrate. Due to the design of these conventional bolometers, the absorber layer is therefore typically unable to completely cover the legs, pillars or the like.
While conventional focal plane arrays having pixel structures with bolometers as described above have been sufficient for most applications in the past, these conventional focal plane arrays may be unable to meet the heightened requirements proposed by a number of future applications that demand increased performance and perhaps decreased size. In particular, as the pixel density and performance requirements of focal plane arrays are increased, the compromises that have been made in the past in order to balance the absorption characteristics of a conventional bolometer become unworkable. In addition, concerns relating to the costs to manufacture focal plane arrays and the size of the resulting focal plane arrays are driving focal plane arrays to be even smaller, thereby correspondingly placing even more importance upon maximizing the fill factor of the pixel structures. For the reasons described above, conventional focal plane arrays having a plurality of pixel structures, each of which includes a bolometer having absorber and transducer supported above the surface of a substrate, do not appear to be able to meet the performance requirements demanded by some modern applications.
One approach designed to overcome most of the aforementioned shortcomings of conventional focal plane arrays is described in U.S. Pat. No. 6,307,194 (the '194 patent) and assigned to DRS Sensors & Targeting Systems, the assignee of the present application. The contents of the '194 patent are incorporated in its entirety herein by reference. The pixel structure of the '194 patent includes a bolometer having a transducer and an absorber that have been spaced-apart, preferably disposed in different planes and separated by a gap so as to permit the transducer and the absorber to be separately optimized. As such, the absorption characteristics of the bolometer can be maximized, while concurrently minimizing the thermal loss to the substrate. This combination of maximizing the absorption characteristics and minimizing the thermal loss, in turn, translates into a more sensitive bolometer and an increase in the thermal resistance of the bolometer. In order to support the absorber in a spaced-apart relationship from the transducer, the bolometer of the '194 patent preferably includes a post extending between the transducer and the absorber. The post is preferably thermally conductive to thereby maintain thermal contact between the absorber and the transducer. As such, the absorber can transfer thermal energy via the post to the transducer in response to the absorbed radiation. In order to further improve the performance of the bolometer, the bolometer can also include a reflector disposed upon the substrate and underlying the transducer.
As disclosed in the '194 patent, since the absorber and the transducer of the bolometer are spaced-apart from one another, the absorber and the transducer can be separately designed in order to optimize the performance of the pixel structure. For example, in order to increase the absorption characteristics of the pixel structure, the absorber is preferably relatively large. In this regard, the absorber and the transducer are preferably sized such that the surface area of the absorber is greater than the surface area of the transducer. As such, the absorber covers the transducer in an umbrella-like configuration. Moreover, in embodiments in which the pixel structure includes legs for supporting the transducer in a spaced-apart relationship with respect to the substrate, the absorber is preferably sized such that the absorber covers both the transducer and the legs in an umbrella-like configuration. By filling most, if not all, of the footprint of the pixel structure with the respective absorber, the focal plane array of the present invention maximizes the fill factor, thereby correspondingly maximizing the percentage of incident thermal radiation captured by the absorber.
While the pixel structure disclosed in the '194 patent addresses many of the shortcomings of conventional focal plane arrays, it is always desirable to further improve upon existing designs. In this regard, certain types of materials used to create the transducer element of many bolometers generally cause some level of a low frequency, 1/f, noise in the signal generated by passing current through the transducer element. For example, while the transducers of some bolometers are formed of vanadium oxide (VOx) since it has an electrical resistance that predictably varies in a significant manner in response to changes in its temperature, VOx generates 1/f noise. Typically, the level of 1/f noise scales inversely with the square root of the volume of the transducer material used in the bolometer. Therefore, smaller area transducers tend to exhibit higher levels of 1/f noise than do larger area transducers. This 1/f noise is a drawback to the focal plane array because 1/f noise generally limits the ability of the infrared detector to effectively calibrate for other types of noise, such as spatial pattern noise, thereby removing these other types of noise from the signal generated by passing current through the transducer element. Therefore, it would be desirable to have a pixel structure that addresses the shortcomings of conventional focal plane arrays and additionally decreases 1/f noise by maximizing the volume of the transducer element in relation to the rest of the pixel structure. The pixel structure should additionally maximize the absorption characteristics of the bolometer, while minimizing the thermal loss to the substrate.